The present applicant developed, and now manufactures and sells, a hybrid car called the HIMR which makes combined use of an internal combustion engine and an electric motor. In this vehicle, a three-phase alternating current squirrel-cage induction machine is coupled to the crankshaft of the internal combustion engine, a large battery is mounted on the vehicle, a bidirectional inverter circuit is coupled between this battery and the squirrel-cage induction machine, and this inverter circuit is controlled by a program control circuit (see WO88/06107).
With this system, when the vehicle accelerates, the rotating magnetic field applied to the squirrel-cage induction machine is controlled so that the squirrel-cage induction machine constitutes an electric motor, and when the vehicle decelerates, the rotating magnetic field applied to the squirrel-cage induction machine is controlled so that the squirrel-cage induction machine constitutes a generator. The control performed by this system is such that the battery discharges when the squirrel-cage induction machine is utilized as an electric motor, and the battery charges when it is utilized as a generator. In other words, this system is controlled to provide regenerative braking.
This system has been mounted on large buses and has been applied to buses on urban routes and to buses operating in regions where it is essential to keep pollution extremely low. In recent years, pollution from the exhaust of vehicles with internal combustion engines has become a major problem, and this has led to discussion of the possibility that most vehicles being run in urban areas will become electric vehicles, despite the higher price of such vehicles and the fact that their fuel is quite expensive.
The HIMR described above is arranged to provide a battery compartment in the vehicle, to use batteries with a terminal voltage of 12 V as unit cells, such batteries being mass produced and therefore inexpensive to procure, and to mount twenty-five of these in the battery compartment and connect them electrically in series to give an overall terminal voltage of 12 V.times.25=300 V. This arrangement is utilized as the battery for supplying energy for running the vehicle.
The term "unit cell" here signifies the unit which, when a multiplicity are connected in series, comprises the battery for supplying energy to run the vehicle. For example, although in the case of a lead storage battery the chemical properties dictate that the terminal voltage of the smallest unit cell is 2 V, a commercial battery generally comprises a plurality of these 2 V cells connected in series and housed in one casing. For example, in the case of a lead storage battery, the terminal voltage of the unit cell can be 2 V, 4 V, 6 V, 12 V, 24 V and so forth. For other kinds of battery, the terminal voltage of the unit cell is determined by the chemical properties of the cell and by the number of cells connected in series.
The present applicant has filed an International Patent Application (see PCT/JP96/00966 and WO96/32651) relating to the monitoring of unit cells.
When a vehicle is started or accelerated using the squirrel-cage induction machine as an electric motor, energy is extracted from the battery and used, and therefore the battery discharges. When a vehicle decelerates and the squirrel-cage induction machine is operated as a generator, regenerative braking takes place and the battery charges. In a battery which repeatedly charges and discharges in this manner, charging and discharging are not necessarily in a state of equilibrium.
Discharging time will be longer when running along a road with many upward slopes, while charging time will be longer when travelling along a road with many downward slopes. Because the type of battery in present use is basically the lead storage battery, consideration has to be given to battery deterioration due to overcharging or over-discharging.
Hitherto, the relevant control has been carried out by measuring the terminal voltage of the unit cells. For example, given a unit cell with a standard voltage of 12 V, if the voltage at which charging is ended is set at 13.2 V and the voltage at which discharging is ended is set at 11.4 V, the unit cell is controlled so that an overcharging warning is displayed and charging automatically stopped if 13.2 V is exceeded, and so that an over-discharge warning is displayed and discharging automatically stopped if the voltage drops below 11.4 V.
However, battery deterioration proceeds during repeated charging and discharging, and the quantity of electricity which can be charged and discharged decreases. Namely, with a battery in which deterioration has progressed, during charging the voltage at which charging is ended will be reached despite the fact that the battery has not been fully charged. During discharging under load, the battery voltage will end up below the voltage at which discharge should be ended. Consequently, if charging and discharging control is performed on the basis of terminal voltage and the state of battery deterioration is ignored, the resulting control will sometimes restrict charging despite further charging being possible, and will sometimes perform further charging despite no more charging being possible.
Furthermore, although a battery for storing energy for driving a vehicle uses a plurality of unit cells connected in series, these series-connected unit cells do not deteriorate uniformly. Instead, there is variability in their deterioration and this increases if uniform charging and discharging are performed.
Hitherto, charging current and discharging current have been controlled on the basis of terminal voltage in the manner described above, and therefore the control has not been of the sort which causes the voltage at which charging is ended and the voltage at which discharge is ended, these voltages constituting the basis for this control, to vary with the deterioration of the unit cells. As a measure for avoiding overcharging or over-discharging of a battery in which deterioration is advanced, the voltage at which charging is ended and the voltage at which discharging is ended are set in advance to suit a battery with advanced deterioration. This means that for a new battery, full use is not made of its storage capacity.
By performing repeated experiments, the present inventor has been able to obtain a large number of records relating to the running and maintenance of the HIMR vehicles mentioned above. A battery gradually deteriorates with repeated charging and discharging, and therefore has to be replaced after a certain time. However, a careful study of the aforesaid maintenance records has shown that even for buses operating on routes involving relatively uniform driving conditions, battery life is by no means uniform and instead exhibits large variability. It was also noticed that although charging and discharging were carried out on a large number of series-connected unit cells, each unit cell under these conditions had individual characteristics, and despite the series connection charging and discharging were not carried out uniformly.
This will be explained in detail. If for example twenty-five unit cells are discharged when connected in series, energy is not released uniformly from all twenty-five unit cells. Furthermore, when charging series-connected unit cells, not all the cells are charged uniformly. This is easily understood in terms of electrical characteristics by assuming that the internal resistance (R) of the individual unit cells is not equal. The series connection means that the current (I) is equal, but during charging and discharging the charging or discharging energy per unit time (I.sup.2 R) will not be equal. A unit cell with a higher internal resistance will have a higher terminal voltage during charging than the other unit cells, and conversely will have a lower terminal voltage during discharge than the other unit cells. Even assuming that the terminal voltages are actually equal, if all the unit cells are repeatedly charged and discharged at a standard or rated voltage, a cell with a high internal resistance will end up being overcharged during charging, with the result that this unit cell alone will undergo accelerated deterioration. Furthermore, despite being charged and discharged on the basis of series connection, a unit cell with a high internal resistance will acquire an elevated cell temperature, with the result that its characteristics will differ from those of the other unit cells, and again this unit cell alone will end up deteriorating before the other unit cells.
The inventor has carried out various trials such as housing unit cells from the same production lot in one battery compartment. It was discovered that even if the characteristics of the unit cells in a new vehicle are uniform, when the vehicle has been used for a long period of time these characteristics exhibit variability, and non-uniform deterioration accelerates. In general it is not individual unit cells of a battery which are replaced, but rather all the unit cells are replaced simultaneously. This is clearly a cause of shortened battery life. Moreover, the use and subsequent disposal of large numbers of batteries will constitute a new source of pollution.
The maintenance and inspection of unit cells should therefore involve measuring the voltage and current of each unit cell after connection of a suitable load, and then, for each unit cell and in accordance with the state of deterioration of that cell, taking appropriate action aimed at avoiding variability in the characteristics of the unit cells.
In the prior art, the maintenance and inspection of a high-voltage battery of this sort requires that measuring equipment be connected to cell terminals. Even when it is thought that just one or two of a large number of series-connected unit cells have become defective, the maintenance and inspection of a conventional high-voltage battery has necessitated connecting measuring equipment to the terminals of each and every unit cell and making a large number of measurements in order to discover which unit cells are defective. It is therefore stipulated that the maintenance and inspection of a high-voltage battery of this sort has to be carried out by someone who has received a prescribed training and who carefully employs a stipulated safe procedure. This means that maintenance and inspection cannot be performed freely by the driver and that each vehicle has to be taken to a designated vehicle service center, etc.
The present situation is therefore that although it is important from the point of view of avoiding breakdowns for the driver of a vehicle to inspect the battery either before or after operating the vehicle, or while driving, in the case of a high-voltage battery this is usually impossible for the driver to do.
Against such a background, it is an object of the present invention to provide a control system for a vehicle-mounted battery which is capable of making full use of the storage capacity of the battery. It is a further object of the present invention to provide an arrangement for adaptively controlling the terminal voltage limit during charging in accordance with the state of deterioration of the battery. It is yet another object of the present invention to provide an arrangement for adaptively controlling the terminal voltage limit during discharging in accordance with the state of deterioration of the battery. It is a further object of the present invention to provide a control system for a vehicle-mounted battery which is capable of increasing battery life. It is still another object of the present invention to provide an arrangement which, for a battery comprising a plurality of series connected unit cells, can control the state of deterioration of each of these unit cells.
It is an object of the present invention to provide an arrangement which can facilitate maintenance and inspection. It is another object of the present invention to provide an information transmission arrangement which simplifies battery maintenance. It is yet another object of the present invention to provide an arrangement whereby maintenance personnel can make measurements without coming into contact with live parts of the battery. It is a further object of the present invention to provide an arrangement whereby the state of deterioration of the battery can be found while the battery is in use. It is yet another object of the present invention to provide an arrangement whereby the state of a battery mounted on an electric vehicle can be detected while the vehicle is travelling.